1. Field of the Invention
This invention relates to the art of making electron emission surfaces sensitized by exposure to alkali metals and more particularly to the preparation of such surfaces for increased red photosensitivity.
2. Description of the Prior Art
Methods of sensitizing electron emissive surfaces of, for example, supported base layers of antimony are well known in the art of electron discharge devices. Photoemissive materials and techniques relating thereto are, for example, described in Photoemissive Materials by A. H. Sommer, John Wiley and Sons, Inc., New York, 1968 and is herein incorporated by reference.
In the manufacture of photomultipliers, it is desirable to have a photocathode which is highly sensitive to visible and near infrared light and which provides a high signal to noise ratio. In general, two procedures have been utilized which lead to electron emissive electrodes of increased photosensitivity. The first is the process of superficial oxidation and the second is the use of a relatively thin manganese oxide, both of which are disclosed in Photoemissive Materials and in U.S. Pat. No. 4,002,735 issued to McDonie et al on Jan. 11, 1977.
In a photocathode, for example, comprising an evaporated film of antimony sensitized by exposure to cesium, a significant increase in cathode sensitivity has been measured after introduction of controlled quantities of oxygen. The introduction of the oxygen primarily reduces the surface barrier of the cathode, resulting in increased response to all wavelengths, longer threshold wavelength and a lower photoelectric work function. However, it has been determined that there is a level at which further exposure to oxygen reduces the cathode sensitivity.
It has also been found that the chemical nature of the electrode may affect the photoemissive properties of a photocathode. In phototubes, a cesium-antimony cathode, for example, is often deposited on a solid metal substrate, usually made of a metal suitable for use in vacuum tubes, such as nickel. Frequently the antimony film is evaporated onto the substrate before the substrate is mounted in the tube and hence before degassing. It has been observed that at degassing temperature above 265.degree. C. significant alloying of nickel occurs with the antimony. An efficient photocathode cannot be produced with the antimony-alloy electrode since there is insufficient antimony available for a reaction. Various techniques have been adopted to avoid or minimize the alloy formations, permitting higher temperatures for degassing. Bake out temperatures, however, must remain below 285.degree. C. with antimony electrodes, since the antimony will evaporate above this temperature in a vacuum.
One method of preventing alloying is the adjustment of the antimony thickness in such a way that the substrate metal is not diffused into the pure antimony within the escape depth of photoelectrons. Another technique of alloy prevention is the deposition of an intermediate layer between the nickel substrate and the antimony layer. The material of such an intermediate layer widely used is manganese oxide, such as referred to previously in U.S. Pat. No. 4,002,735. Manganese oxide is often utilized in photocathodes since not only does it prevent alloy formation but also tends to produce cathodes with higher quantum yield and a longer threshold wavelength, qualitatively similar to the effects produced by superficial oxidation, in particular in semitransparent cesium-antimony cathodes. Although the manganese oxide appears to have a specific effect on the photoemissive properties of the cathode, this effect has not been found to be directly attributable to the amount of oxygen available. It has been experimentally concluded that the oxidation time of the cathode is independent of the thickness of the manganese oxide. (See Photoemissive Materials, page 74)
Establishment of definite relationships of the chemical nature of photoemissive surfaces, such as that between oxidation time and the thickness of an intermediate film, is desirable and advantageous since the effects on photoemissive properties may be more readily determined by these relationships. Such relationships could be utilized to achieve photocathodes of improved sensitivity since the factors which affect the photoemissive properties could be more effectively controlled.